Lyman Alpha Forest

There are clouds of gas between distant
quasars and the Earth that absorb
ultraviolet light at the wavelength of the Lyman alpha line of
hydrogen at a wavelength of 122 nm.
Quasars also emit a strong Lyman alpha emission line.
But the absorbing clouds all have smaller
redshifts than the quasar
since they have smaller distances. As a result the absorption lines
are all on the blue or shorter wavelength side of the quasar emission
line.

The cartoon above shows a quasar with its Lyman alpha emission line
redshifted from the ultraviolet into the red, and the Lyman alpha
absorption lines from four intervening clouds appearing as orange,
yellow and green-blue.

The figure below shows two actual quasar spectra. One is the nearby
quasar 3C273 while the other is a large redshift object. This figure
was adapted from Bill Keel's
web site.

The clouds that produce the absorption lines are much more numerous at
high redshifts so a very dense array of absorption lines results: the
"forest".

We know that there are a small number of very big clumps of hydrogen in
the distant Universe: the galaxies. We also know that smaller galaxies, the
dwarf galaxies, are very much more common. Most of the clouds in the
Lyman alpha forest are much less massive than dwarf galaxies and these
small clouds are much more numerous. We can only see these very low
mass clouds by the absorption they produce in the strongest line of the
most abundant element: Lyman alpha. Thus by studying the Lyman alpha forest
we can learn about the density fluctuations in the Universe on the smallest
observable scales.

Note that if Arp were correct and quasars had a redshift much larger than
the redshift due to their distance, then there should be a gap on the
blue side of the Lyman alpha emission line before the absorption lines
began. Such gaps are not seen.
So if Arp were correct the Lyman alpha forest would have to be an intrinsic
property of the quasar, which would be a very unlikely situation. Distant
galaxies are seen which also show the Lyman alpha forest, so we know that
the intervening clouds do exist. For Arp to be correct the intrinsic
absorption lines would have to act exactly like the intervening clouds would
act under the standard hypothesis that the quasar redshift is entirely
cosmological.